The invention relates to an outlet device for a melting crucible for a vacuum casting installation for small parts comprising a melting crucible with a bottom outlet opening in the form of a cylindrical bore and a stopper for closing the bottom outlet opening.
Vacuum casting installations for small parts are known in which in particular precious metals and light metals are cast. In these known installations a collecting volume for molten material is disposed above the mold and this collecting volume can be part of the melting crucible or can be fed via an inflow channel. In the case of the implementations in which the collecting volume is a part of the melting crucible, the melting crucible is often equipped with an induction heater. On the bottom surface of the melting crucible a bottom outlet opening is disposed which is closable with a stopper. The stopper is therein guided through the collecting volume for the molten material in the melting crucible and is movable in the direction of the longitudinal axis of the melting crucible. When casting precious metals, only that quantity of raw material is charged into the melting crucible which is necessary for filling a casting mold. The material is melted in the melting crucible with the stopper closed and, when the desired casting temperature is reached, is poured off by opening the stopper. The known combinations of stopper and bottom outlet openings forming the outlet device are therein implemented such that they ensure good sealing during the melting process. For this purpose the front end of the stopper is implemented semispherically or conically. The beginning of the bottom outlet opening directed toward the collecting volume for the molten material is either sharp-edged or provided with an oblique conical surface.
With this implementation of the bottom outlet opening and of the front end of the stopper difficulties are encountered in practice since, after the opening of the stopper, the liquid jet of material flowing out becomes detached at the inlet edges of the bottom outlet opening and generates eddies, jet pinching and flow separation. As a consequence the outflow rate and the outflow quantity are reduced and the time for the flowing-out of the required quantity of material into the casting mold is thereby prolonged. In addition, the danger exists that through the turbulences gas is entrained and included in the liquid material and material particles from the walls of the bottom outlet opening are entrained and impurities form. Due to the extension of the casting time, the solidification process is not uniform in all regions of the casting mold and different crystallization structures and thus faults in the poured-off object can form. In addition, the disadvantage exists that through the turbulences and detachments metal particles, solidified at the end of the casting process, accumulate in the wall regions which hinder the subsequent casting process or disturb the sealing between stopper and bottom outlet opening. This results in the necessity of having to move the stopper out of the melting crucible after every casting process and checking and cleaning the bottom region of the collecting volume for the molten material and the outlet opening in order to be able to carry out the subsequent casting process without disturbances. This necessitates additional operating expenditures and loss of time which reduces the economy of such installations.